1. Field of the Disclosure
The present disclosure relates to a production method of an optical fiber preform and a production method of an optical fiber.
2. Description of the Related Art
Commonly, silica-glass-based optical fibers are produced by drawing an optical fiber preform made of silica glass. For production methods as such for optical fiber preforms, a vapor-phase axial deposition (VAD) method, an outside vapor-phase deposition (OVD) method, a modified chemical vapor deposition (MCVD) method, a plasma method, or the like is used widely.
In these production methods, optical fiber preforms are produced by using, for example, silicon tetrachloride as a material for the silica glass and making the silicon tetrachloride be subjected to hydrolysis reaction or oxidation reaction.
An optical fiber preform includes a column-shaped parallel portion having an approximately constant outer diameter and tapered portions connected to an upper end and a lower end of this parallel portion. The upper-end tapered portion expands its outer diameter downwardly and gradually to be connected to the parallel portion, and the lower-end tapered portion expands its outer diameter upwardly and gradually to be connected to the parallel portion.
Meanwhile, the tapered portions provided at the end portions of the optical fiber preform are vulnerable to cracking for reasons that the density of deposited silica particles tends to lower, stress tends to occur in a thermal treatment such as a dehydration step, a sintering step, or the like. Therefore, measures for restraining the optical fiber preform from being cracked are provided to the tapered portion in some cases (see Japanese Patent Application Laid-open Publication No. 2010-37125).
Although it is possible to restraining production of a crack by increasing thermal shrink at the tapered portion to increase the density of the silica particles, an attempt of increase in thermal shrink encounters a limitation that another problem occurs such as decentralization of a core or the like.
As described above, the measurement for restraining cracking at the tapered portion is not so sufficient that cracking still occurs at the tapered portion sometimes. To address this, a more effective countermeasure for cracking is required.
There is a need for a production method of an optical fiber preform and a production method of an optical fiber being able to restrain a cracking from being produced.